This invention relates to navigation receivers and, particularly, receivers for use in the global position system (GPS), a satellite-transmitted navigation signal system.
The GPS is being deployed by the United States government; when fully implemented it will consist of up to 24 satellites in precise orbits uniformly distributed over the globe providing continuous, world-wide, all-weather navigation information to all users equipped with a GPS receiver.
All GPS satellites broadcast at an identical carrier frequency, but each has a unique Pseudo-Noise (PN) code which modulates (multiples) the carrier. Present codes consist of 1023 consecutive chips of value +1 or -1 and approximately 1 .mu.sec duration. The exact duration is such that 1023 consecutive chips take up exactly 1 msec. This is referred to as an epoch (code epoch). The uniqueness of the code manifests itself in the manner in which the pattern of +1s, -1s are arranged. The pattern repeats itself once every 1023 chips, or equivalently, every 1 msec.
Each satellite transmits data at a 20 msec/bit rate. Data bit boundaries are synchronized with epoch boundaries and occur once every 20 epochs. Data is differentially encoded. This means that a 1 is represented by the presence of 180.degree. phase transition (sign reversal) at a data boundary and a 0 by the absence of such a transition.
The strength of the received signal is nominally -160 dBW (10.sup.-16 below 1 watt). Because of losses and thermal effects the signal to noise ratio at the output of the antenna--preamplifier assembly is about -33 dB, i.e. the signal is 2000 times weaker than the background noise.
Signal detection is impossible unless the signal to noise ratio is somewhat in excess of 10 dB i.e. the signal is about 10 times stronger than the background noise. The initial task is to boost the signal without also amplifying the noise. The method used in the GPS system to achieve this end is referred to as "a spread spectrum" technique.
In its GPS implementation the satellite signal (in each satellite transmitter) is multiplied by a known unique sequence of +1, -1 amplitude pulses each lasting 1 .mu.sec. This is the PN code referred to above. The effect of this operation is to spread the signal over a bandwidth equal to the reciprocal of the pulse duration 1 MHz in this case.
By multiplying the received signal by an exact replica of the code initially used, each +1 is multiplied by a +1 resulting in +1 and each -1 by a -1 also resulting in a +1. This process is referred to as correlation and effectively removes the initial modulation. However, since the PN code itself has a periodicity of 1 msec, the bandwidth of the signal after correlation is 1 kHz. Correlation therefore results in a bandwidth compression of a 1000 to 1 (30 dB). This results in a post-correlation signal to noise ratio of -3 dB. The remaining 13 or so dB needed to make the signal to noise ratio exceed 10 dB, and thereby render the signal detectable, are obtained by repeating the correlation process with the constantly incoming received signal.
In addition to the above, the signal will be degraded by unknown frequency offsets produced as a result of relative satellite and receiver motion together with a number of other minor causes. These are usually referred to as doppler offsets. Assuming there are M satellites visible at a particular time, the total signal as seen by the antenna of the receiver may be represented by ##EQU1## where m.sub.i --PN code,
.tau..sub.i --unknown code phase (a multiple of 1 .mu.sec, from 0 to 1022), PA1 d.sub.i --data sequence, PA1 .omega..sub.C =2.pi.f; f=carrier frequency, PA1 .omega..sub.Di =2.pi.f.sub.Di --unknown doppler, PA1 .psi..sub.i --unknown signal phase, and PA1 t--time
Although all PN codes for all satellites are known, one does not know, to start with, which satellites are currently visible and, of those that are visible, which of the 1023 possible code positions (phases) are being currently received. Moreover, if the unknown Doppler offset is excessive (larger than .+-.500 Hz) it will totally eliminate any processing gain the band-spreading techniques can provide. It is therefore necessary to determine the phases of the incoming codes and the doppler offset to within .+-.500 Hz before signal acquisition is possible.
Thus, the user element in the GPS must perform a variety of tasks, viz: it must identify the visible satellites; it must acquire signals from a sufficient number of the visible satellites to obtain pseudo-range information; it must track the acquired signals as the relative positions of the receiver and the satellites charges; and it must demodulate and convert the data to yield navigational information.
There exists a need for a receiver to perform the tasks identified above. Acquisition speed, accuracy and simplicity are among the principal criteria for a GPS receiver.